1. Field of the Invention
The present invention relates to a power cross detection circuit which senses a power cross condition using a loop current and a loop voltage.
2. Background of the Related Art
A terminal connected to a telephone line should periodically undergo a "power-cross" test. This test is performed in order to verify the ability of the terminal to detect the presence of a foreign potential, such as a high voltage of up to 1000 volts AC from a local power company or the like, which may become crossed with the telephone line in an environment outside of the telephone plant. FIG. 1 shows a telephone 10 connected to a telephone plant 12 through wires 14. A power line 16 poses a threat to the telephone plant 12 and any personnel operating the equipment that are connected to the wires 14 that are crossed with the power line 16.
The power-cross typically involves forcing a 1 ampere current into tip and ring lines 18, 20 regardless of the state (i.e., talking, standby or ringing) of the lines, and checking whether the telephone plant 12 survives the test. Generally, in order to provide a 1 ampere current, a 1,000 volt, 60 hertz signal through a 1k ohm resister is applied to the tip and ring lines 18, 20. Other combinations of voltages and resistance values, such as a 600 volt signal through a 600 ohm resistor, or a 50 volt signal through a 50 ohm resistor, could also be used to obtain the 1 ampere current.
As mentioned above, the power-cross test is applied regardless of the state of the telephone plant 12. For example, a person could be talking on the telephone 10, or the telephone 10 could be in a standby mode, or the telephone 10 could be ringing and on-hook. In conventional techniques, during either the talking mode or the standby mode, as shown in FIGS. 1 and 2, a protection device 22, such as a diode or a neon lamp, is connected to ring and tip lines 18, 20 via relays (not shown). When a high voltage, such as an AC power signal, crosses wires 14 (FIG. 1), the current goes through the protection device 22, thereby preventing damage to the telephone plant 12 or telephone 10.
As shown in FIG. 3, during the ringing mode, the telephone 10 is connected to ground GND via the tip line 18 and relay S1A, and the ring line 20 is connected to a ringer voltage RVbyrelays S1A, S1B in order to perform the telephone ringing operation. In this situation, the protection device 22 is disconnected. Thus, although there is some kind of protection provided in the talking and standby modes, there is no protection provided if a power-cross condition occurs during the ringing mode.